Uphill–in both directions–through at least 2 feet of snow…
I rode my skateboard downhill to and from school every day. The high temperature was 85F with 30% humidity every day. Charmed life.
I have actually gotten very good gas mileage on hilly roads. As long as the down grades aren’t so steep that you have to ride the brakes to control the speed of the car. Unlike brakes, which turn kinetic energy into unrecoverable heat, upgrades convert kinetic energy into potential energy which is returned to the car when the car goes back downhill.
The trick is to manage your speed so that you don’t have to scrub off your car’s energy with braking. That means anticipating downgrades ahead and holding off accelerating to cruising speed, allowing the grade to do the acceleration for the engine. If you hit the grade already at full cruising speed, you end up having to ride the brakes to keep going over a safe speed. Even if the road is so straight that you can safely go 80 at the bottom of the hill when not braking, just going 80 causes the car to lose a lot of energy to air resistance.
I drove my 2002 Sienna 223,000 miles, often on the same long trips. At 70 mph, it consistently got 24 mpg. At 60 to 65 in Eastern states, it ran over 26 mpg.
Once, when I had tire balance problems and did not drive over 50 on Mexican highways, it ran around 30 mpg.
The brothers used to say for maximum mpg, run at the speed where it has just shifted into Overdrive, or whatever you want to call it. Of course, if you are in hilly country, you might have to change that speed.
Tire pressure also makes a huge difference. I went from 32 to 28 PSI for a while and MPG went way down, by about 2-4 MPG.